The present invention relates to a process control technique and, more particularly, to a control apparatus having a limit cycle auto-tuning function of outputting a manipulated variable with a constant amplitude to an object to be controlled and adjusting a control parameter on the basis of a control response corresponding to the controlled variable output.
A general-purpose temperature controller or the like has an auto-tuning (self-tuning) function in order to easily complete adjustment of PID parameters. A typical method of the auto-tuning function is a limit cycle auto-tuning method of setting upper and lower limit values for a manipulated variable MV to be output to an object to be controlled, generating a limit cycle having a constant manipulated variable amplitude, and adjusting PID parameters (reference: Kazuo Hiroi, “Fundamentals and Applications of Digital Instrumentation Control System”, Kogyougijutsusha, ISBN4-905957-00-1, pp. 156-159, October 1987).
An example of limit cycle auto-tuning will be described. First of all, a manipulated variable lower limit set point OL_AT which designates the lower limit value of the manipulated variable MV to be output to an object to be controlled in executing limit cycle auto-tuning, and a manipulated variable upper limit set point OH_AT which designates the upper limit value of the manipulated variable MV are set in advance.
In executing limit cycle auto-tuning, a controlled variable PV and set point SP are compared (step S401 in FIG. 7). If the controlled variable PV is larger than the set point SP, the lower limit value OL_AT of the manipulated variable MV is output to the object (step S402). If the controlled variable PV is equal to or smaller than the set point SP, the upper limit value OH_AT of the manipulated variable MV is output to the object (step S403).
Extreme value increment/decrement detection processing of detecting the extreme value of the controlled variable PV is performed (step S404). Processes in steps S401 to S404 are performed every control cycle. If four extreme values of the controlled variable PV are detected, detection ends. A deviation Er between the set point SP and the controlled variable PV is given byEr=SP−PV  (1)
As shown in FIG. 6, a first extreme value deviation Er1 represents a deviation in the latest extreme value out of four detected extreme values; a second extreme value deviation Er2, a deviation in the second latest extreme value; and a third extreme value deviation Er3, a deviation in the third latest extreme value.
A first manipulated variable switching elapsed-time Th1 is a time interval between time t5 at which the sign of the deviation Er is reversed immediately before the first extreme value deviation Er1, and time t6 at which the first extreme value deviation Er1 is obtained. A second manipulated variable switching elapsed-time Th2 is a time interval between time t3 at which the sign of the deviation Er is reversed immediately before the second extreme value deviation Er2, and time t4 at which the second extreme value deviation Er2 is obtained.
PID parameters including a proportional band Pb, integral time Ti, and derivative time Td are calculated by equations (2) to (4). The calculated PID parameters are set in the controlling element of the control apparatus (step S405).Pb=250|Er2−Er1|/(OH—AT−OL—AT)  (2)Ti=6(Th1+Th2)  (3)Td=1.2(Th1+Th2)  (4)
Thereafter, limit cycle auto-tuning ends.
Some general-purpose temperature controllers directly use an upper limit set point OH and lower limit set point OL of the manipulated variable MV that are output to an object to be controlled during actual control, as the manipulated variable upper limit set point OH_AT and manipulated variable lower limit set point OL_AT which are used to execute limit cycle auto-tuning. In general, OH=100% and OL=0%. In auto-tuning, therefore, the manipulated variable upper limit set point OH_AT=100%, and the manipulated variable lower limit set point OL_AT=0%.
For a heat-insulating object to be controlled, the manipulated variable MV necessary to maintain the controlled variable PV around the set point SP is as low as MV=20% or less. In this situation, if auto-tuning at MV=0% to 100% is executed with the setting of OH_AT=100%, the temperature rises quickly and drops slowly (the object is hardly cooled because of a high heating insulating property). Compared to the case of FIG. 8A in which limit cycle auto-tuning is properly executed, the limit cycle takes a long time, as shown in FIG. 8B.
In conventional limit cycle auto-tuning, it is also possible to change the manipulated variable upper limit set point OH and manipulated variable lower limit set point OL to proper values, and execute auto-tuning. If, however, the manipulated variable upper limit set point OH and manipulated variable lower limit set point OL are changed in executing auto-tuning, they must be reset at the end of auto-tuning.
In particular, an object to be controlled which must be frequently auto-tuned by the operator on site frequently requires cumbersome setting operation, greatly decreasing the workability.
The manipulated variable upper limit set point OH and manipulated variable lower limit set point OL are not always set to OH=100% and OL=0%, and may be set to other values. In the worst case, the operator on site may set the manipulated variable upper limit set point OH_AT used to execute limit cycle auto-tuning to be higher than the manipulated variable upper limit set point OH during actual control and the manipulated variable lower limit set point OL_AT used to execute limit cycle auto-tuning to be lower than the manipulated variable lower limit set point OL during actual control. To the contrary, the operator may erroneously set the manipulated variable upper limit set point OH and manipulated variable lower limit set point OL in resetting them.